A conductive polymer has a high conductivity so that it can be used as a solid electrolyte solid of an electrolytic capacitor such as aluminum solid electrolytic capacitor, tantalum solid electrolytic capacitor and niobium solid electrolytic capacitor.
With regard to the conductive polymer for the application, ones obtained through chemical oxidative polymerization or electrolytic oxidation polymerization of polymerizable monomers such as thiophene or its derivatives can be used.
As dopants for performing chemical oxidative polymerization of thiophene or its derivatives as a polymerizable monomer, organic sulfonic acids can be mainly used. Among them, aromatic sulfonic acids are said to be favorable. As oxidants, transition metals can be used. Among them, ferric compounds are said to be favorable. Usually, ferric salts of aromatic sulfonic acids can be used for an agent serving as oxidant and dopant for the purposed of chemical oxidative polymerization of thiophene or its derivatives.
In particular, among the ferric salts of aromatic sulfonic acids, it is said that ferric toluenesulfonate or ferric methoxybenzenesulfonate can be favorably used. See Patent Publications Nos. 1 and 2 below.
However, a conductive polymer, prepared by using ferric toluenesulfonate as an agent serving as oxidant and dopant, is not enough in such properties as initial resistance and heat resistance. Also, a conductive polymer, prepared by using ferric methoxybenzenesulfonate as an agent serving as oxidant and dopant, is better than the conductive polymer using ferric toluenesulfonate in the initial resistance and heat resistance, but it is still is not fully satisfied with.
This is because ferric toluenesulfonate and ferric methoxybenzenesulfonate are solid and they are generally used in a condition of an alcohol solution, but precipitates generate during storage.
Therefore, an alcohol solution of ferric toluenesulfonate or ferric methoxybenzenesulfonate, having precipitated, becomes lower in the homogeneousness. Thus, the solid electrolytic capacitor using thereby obtained conductive polymer, i.e., a solid electrolyte, results in lowering the ESR (equivalent series resistance) and high temperature stability.
Also, it was reported that by constituting a ferric salt of a certain benzenesulfonic acid derivative as an agent serving as oxidant and dopant and by lowering the water content therein, a conductive polymer having lowered initial resistance and excellent in the heat resistance can be obtained. See Patent publication No. 3 below.
However, such a method cannot produce a conductive polymer fully satisfied with required properties. This is because the oxidizing power of the agent serving as oxidant and dopant is too strong oxidative power to generate a side reaction.
Patent Publication No. 1: JP laid-open patent publication No. 2003-160647
Patent Publication No. 2: JP laid-open patent publication No. 2004-265927
Patent Publication No. 3: JP laid-open patent publication No. 2007-023090